What are the format specifiers allowed in iOS DateFormatter dateFormat fromTemplate?

Question

I cannot find a list of the format specifiers allowed in the template argument of DateFormatter.dateFormat(fromTemplate:options:locale:).

1. dateFormat directs me to...
2. Date and Time Programming Guide. None of those pages lists the format specifiers. So I checked the suggested related documentation...
3. Date and Time Programming Guide for Core Foundation doesn't have a list.
4. Data Formatting Guide has a section title "Use Format Strings to Specify Custom Formats" but it just points me back to (1) above. None of the other pages in Data Formatting Guide have the format specifiers.

Does anyone know where Apple documents these specifiers?

Answer 1

I'll expand on the answer in the comment with some examples. For iOS 7 and later the format codes are here: http://www.unicode.org/reports/tr35/tr35-31/tr35-dates.html#Date_Format_Patterns. The table is too big to copy it all here but here's a few that helped me get started. See that link for all of the formats and explanations.

• era: G (AD), GGGG (Anno Domini)
• year: y (1984), yy (84), yyyy (1984)
• month: M, MM, MMM, MMMM, MMMMM. Also: L
• day of month: d, dd
• day name of week: E, EEEE, EEEEE, EEEEEE

Here's a playground fragment that I found helpful to explore these.

import Foundation

let components = DateComponents(
    calendar: Calendar(identifier: .gregorian), 
    timeZone: nil, 
    era: 1, 
    year: 1984, 
    month: 1, 
    day: 2, 
    hour: nil, minute: nil, second: nil, 
    nanosecond: nil, weekday: nil, 
    weekdayOrdinal: nil, quarter: nil, 
    weekOfMonth: nil, weekOfYear: nil, 
    yearForWeekOfYear: nil)
let aDate = Calendar(identifier: .gregorian).date(from: components)!
let en_US = Locale(identifier: "en_US")
var df = DateFormatter()
func formatdate(_ template: String) -> String {
    let custom = DateFormatter.dateFormat(fromTemplate: template, options: 0, locale: en_US)
    df.dateFormat = custom
    return df.string(from: aDate)
}

formatdate("Mdyyyy") // "1/2/1984"
formatdate("yyyyMMdd") // "01/02/1984"
formatdate("yyyyMMMdd") // "Jan 02, 1984"
formatdate("yyyyMMMMdd") // "January 02, 1984"
formatdate("yyyyMMMMMdd") // "J 02, 1984"
formatdate("yyyyG") // "1984 AD"
formatdate("yyyyGGGG") // "1984 Anno Domini"
formatdate("yyyyMMMddE") // "Mon, Jan 02, 1984"
formatdate("yyyyMMMddEEEE") // "Monday, Jan 02, 1984"
formatdate("yyyyMMMddEEEEE") // "M, Jan 02, 1984"

formatdate("MdYYYY") // "1/2/1984"
formatdate("YYYYMMdd") // "01/02/1984"
formatdate("YYYYMMMdd") // "Jan 02, 1984"
formatdate("YYYYMMMMdd") // "January 02, 1984"
formatdate("YYYYMMMMMdd") // "J 02, 1984"
formatdate("YYYYG") // "1984 AD"
formatdate("YYYYGGGG") // "1984 Anno Domini"
formatdate("YYYYMMMddE") // "Mon, Jan 02, 1984"
formatdate("YYYYMMMddEEEE") // "Monday, Jan 02, 1984"
formatdate("YYYYMMMddEEEEE") // "M, Jan 02, 1984"